1.) The phosphate buffer system is very important for maintaining the pH of the cytoplasm of all cells. Phosphoric acid is a triprotic acid; however, the relevant equilibrium in the biologically useful, neutral range, with pKa of 6.86 , is that of dihydrogen phosphate and monophosphate ions (data from Table 2.6)

ReactantsProductspKa
2.14
6.86
12.4

Using the Henderson-Hasselbalch equation, calculate the pH of a solution containing 0.042 M and 0.058

Solution

The acid (HA) in this mixture is , and the conjugate base (A-) is

Substituting these values into the Henderson-Hasselbalch equation:

2.) An undergraduate student is making 300 mL of 1 M potassium phosphate buffer solution with a pH = 6.96 using 1 M and 1 M . How many milliliters of 1 M will be needed to make this solution?

Solution

Entering the known values into the Henderson-Hasselbalch equation gives:

So we need to solve for the "mole ratio", aka the part.

Take the anti-log of both sides,

 

Next we need to find the total mols in solution. It was given as 300 mL of 1 M solution.

 

Since we are interested in the amount of we will now write the ratio as :

 

Now we just solve for , notice how we can take out a "1 X" from the right side of the equation.

 

Finally, we only need to convert mols back into mL using the molarity/concentration provided.